Rising oil prices and global climate changes are driving efforts to phase out carbon dioxide-emitting, energy inefficient internal combustion-powered vehicles in favour of cleaner and more energy-efficient vehicles. Such eco-friendly vehicles may include electric vehicles and electric hybrid vehicles incorporating electric motors.
Conventional electric motors are generally single-speed machines. Improvements in digitally controlled power electronics have led to the emergence of electric motors whose speeds may be digitally controlled. Moreover, such machines can act as a generator through regenerative braking.
The ability of electric motors to alternate between motoring and regenerative braking functions, combined with their relatively wide speed range, may allow for the delivery of electric vehicles with similar performance as internal combustion-powered vehicles, but with improved energy efficiency and a reduced carbon footprint. However, even with such advantages, current electric vehicles cannot compete with internal combustion-powered vehicles in range of speed, weight and cost, in part because of the substantial increase in the energy storage (i.e. the battery pack) required for electric vehicles.
One way to make electric vehicles a more attractive option for consumers is to reduce the cost and complexity of the electric drive train. Typically, there is a desire for electric drive trains need to satisfy various performance objectives, which may include: high starting and low speed climbing torques, high regenerative and non-regenerative braking torques, high overall power conversion and energy efficiencies, and low motor noises and torque ripples at higher speeds. However, to satisfy such objectives, electric drive trains often require complex, bulky, heavy and inefficient speed reduction gears. This in turn makes it difficult to free up space for a larger electric vehicle battery.
Conventionally, automobiles are designed to have the minimum power engine or electric motor and drive train that can meet the desired performance envelope. Take for example, a 1400 kg passenger car with design objectives of accelerating from 0-60 mph (approximately 0-100 km/h) in 8 seconds, maintaining 80 mph (approximately 130 km/h) steady speed at 6% grade, and providing a total braking deceleration of 0.8 g0 (7.8 m/s2). An engine or motor sized for this specification would be rated for 160 hp (approximately 120 kW) to meet the 0-60 mph acceleration design objective, and 50 hp (approximately 37 kW) to meet the 6% grade design objective. Consequently, a 160 hp engine or motor is typically selected since it satisfies all of the power train objectives. However, this is not the most energy efficient solution, since the peak power for acceleration is typically needed less than 1% of the time and the 6% grade is encountered infrequently except in a mountainous area. The vehicle would also need to include a separate large and powerful mechanical braking system.
A hybrid vehicle aims to address the foregoing problems by incorporating both an internal combustion engine and an electric motor. This allows the hybrid vehicle to use a smaller engine, relying on the electric motor to provide the extra performance boost when desired. For the design specification of the 1400 kg passenger car in the above example, the electric vehicle's internal combustion engine would handle the 6% grade while the electric motor would handle the 0-60 mph acceleration. However, it is a challenge to supply an electric motor to provide a rated 110 hp or 81 kW (for example), to provide the performance boost desired. Instead, the electric motor is generally derated to 50 hp or 37 kW, for example, and a super capacitor and a boost converter are used to turbo charge the motor to 110 hp or 81 kW for the brief period needed to achieve the 0-60 mph acceleration design objective. This is accomplished by energizing the motor stator winding with sufficient current to momentarily increase the torque and power, since both are proportional to the stator phase current in a magnetic flux saturated core. Although a hybrid vehicle is fueled entirely by petroleum fuel, its ability to run the undersized engine more efficiently at or near its optimal speed range most of the time, together with the regenerative braking capabilities and load balancing between the engine and motor, make it a more fuel efficient vehicle than a conventional petroleum internal combustion-powered vehicle.
Current hybrid vehicles still generally rely on the use of two different power trains, using mechanical and electromechanical switching and coupling to combine the power trains. Thus, hybrid vehicles can be complex to manufacture, making them generally more expensive than internal combustion-powered vehicles.
Electric vehicles and electric hybrid vehicles can provide regenerative braking capabilities. Typically, regenerative braking cannot provide braking forces similar to those provided by engine braking in conventional internal combustion-powered vehicles. In electric or electric hybrid vehicles, regenerative braking generally provides only about a tenth of the full braking torque desired. Conventional friction brakes (mechanical brakes) are still typically needed to supplement regenerative braking.
There has been some effort in the field of active or regenerative suspension to provide electric motors and generators by interconverting linear motions and circular motions through mechanical means such as a ball screw. Such designs typically have low conversion efficiencies and may generate only enough power to supply the electricity desired for the controllers and the actuators. Hence, these are customarily termed self-powered or zero-powered active suspension systems, which serve to deemphasize the electricity regeneration aspect.
For example, FIG. 1 shows a rotary regenerative shock damper 100 which converts linear vibration energy to electrical energy using a linear to circular motion converter. Damper 100 has a ball screw 107 consisting of a vertical spiral screw shaft or a rack and pinion 103 which is coupled to a sliding nut 102. Nut 102 is in turn connected to a vertical slider 106 with one end connected to the vehicle chassis. Nut 102 is prevented from rotating by slider 106, which forces spiral screw shaft 103 to rotate. The damper includes a coaxial coil spring 104 which provides the suspension force. This type of damper is called a spring-strut type damper. Alternatively, the coil spring or other spring types could be used separately to isolate the suspension function from the damping function. Spiral screw shaft 103 is geared to a planetary gear set 101 to amplify its rotation speed, and the geared up spinning motion is fed to a DC motor/generator 105 (also referred to herein as a motor or generator depending on the context). In the damper mode, generator 105 converts mechanical linear vibrations into electricity. In the actuation mode, motor 105 drives ball screw 107 directly to extend slider 106 in and out against the chassis to dynamically change the suspension geometry of the vehicle.
Several inefficiencies are apparent from the design of this rotary regenerative shock damper 100. First, the ball screw mechanism for converting linear vibrations to circular motion imparts considerable frictional resistance to the movement of nut 102 against spiral screw shaft 103. This makes the linear to rotational conversion inefficient. The inefficiency is further hampered by planetary gears 101 which become inefficient above a certain gear ratio (typically in the order of 30:1 to 100:1 for the suspension RMS speed to the base speed of the generator) to enable the use of a lightweight motor/generator 105. Second, the conversion from linear to rotational motion is not unidirectional, but consists of rapid back-and-forth rotational movements. Due to the inherent rotational inertias of both the rotor of motor 105 and the mechanical converter (planetary gear set 101 and ball screw 107 itself), and the compliance and deflection of the electromechanical chain which introduces significant backlash, such a regeneration mechanism would be unable to capture the higher frequency vibrations which would merely cause the damper drive train to oscillate at various resonance frequencies of the drive train. Third, because of the randomness of the back-and-forth rotation of the rotor of generator 105, the instantaneous rotation speed tends to stay far below the base speed for generator 105. Since generator 105 only becomes efficient when the rotation speed is at least a finite fraction of the base speed due to ohmic heating, the average mechanical to electric conversion efficiency is low. Fourth, the electromechanical drive train is heavy, complex and costly to build. Since a suspension damper adds to both the sprung weight and the unsprung weight of the vehicle, any gain in ride comfort and road handling performance derived from such a damper is compromised by the added unsprung weight. The above criticisms also apply to other regenerative suspension designs which rely on mechanical linear to rotational conversions such as those incorporating rack and pinion types, step-up gear boxes and rotational electric generators.
FIG. 2A shows a direct electromechanical conversion damper or shock absorber 200. Damper 200 directly uses the relative linear motion between a linear magnetic pole stack 220 (shown in FIG. 2B) consisting of an array of alternating permanent magnetic rings 206, 207, and linearly-wound field coil stack 210. Pole stack 220 and field coil stack 210 together provide a linear electromagnetic motor/generator 230 (also referred to herein as a motor or generator depending on the context) which generates power based on Faraday's law of magnetic induction. If linear field windings or coils 203 can be made sufficiently lightweight, damper 200 would in theory be able to respond to higher vibration frequencies, thereby capturing a larger spectrum of the road noises and yielding higher electromechanical conversion efficiencies. However, despite such promises, the inherent lack of magnification of the linear motion makes it difficult to develop a lightweight linear electromagnetic motor 230 to effectively capture relatively small linear motion (i.e. movements with a linear velocity typically below 10 cm/s). Motor 230 is generally effective at capturing linear motion only at very high vehicle speeds and on rough road surfaces. In consequence, damper 200 is unable to convert much of the linear vibrations to electricity, and the corresponding effective damping coefficients are typically lower than those achieved by dampers relying on linear to circular conversion and step-up gearing. Although not as complex as a rotary regenerative shock damper, manufacturing a linear electromagnetic damper 200 is still challenging due to the desirability of mounting a large number of magnetic rings 206, 207 and windings 203 and the extremely small air gaps desirable to pick up minute vertical displacements typical of road irregularities. Further, it is desirable that damper 200 be larger and heavier than a rotary regenerative shock damper to achieve similar damping coefficients, thus compromising its performance.
There is a general desire for apparatus and systems that address and/or ameliorate at least some of the aforementioned problems. For example, there is a general desire to provide an electric motor which can dynamically adapt to supply the instantaneous power train demands of a vehicle but still satisfy high efficiency design objectives. There is a general desire to provide an electric motor that can function as a generator through regenerative braking and provide sufficient braking torque to meet vehicle braking demands. There is a general desire to provide an electric motor that enables regenerative damping of lateral vibrations typically encountered when a vehicle drives on a road.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.